1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image by irradiating the surface of a charged photosensitive material with a laser beam to form an electrostatic latent image and relates to a control method of the image forming apparatus.
2. Description of the Related Art
An image forming apparatus forming an image by electrophotography typically includes a charging unit that uniformly charges the photosensitive surface of the photosensitive drum. The image forming apparatus also includes a latent-image forming unit that forms a latent image corresponding to image information on the charged photosensitive surface, a developing unit that develops the latent image, and a transfer unit that transfers the developed latent image to a sheet of paper. The image forming apparatus sequentially performs image forming while rotating the photosensitive surface of the photosensitive drum.
The latent-image forming unit includes a semiconductor laser that outputs a laser beam resulting from light modulation of the input image information by a laser driver controller. The laser beam is reflected from a polygon mirror in the latent-image forming unit, and the reflected laser beam scans the photosensitive drum to form the latent image.
Such a semiconductor laser has a current-optical output characteristic shown in FIG. 14. The semiconductor laser emits light emitting diode (LED) light when the semiconductor laser is driven at a current Ib lower than a threshold current Ith, and emits a laser beam having an optical output Po when the semiconductor laser is driven at a current Io higher than the threshold current Ith. This current-optical output characteristic is varied with variation between semiconductor lasers and change in temperature.
Accordingly, the latent-image forming unit generally includes an automatic power control (APC) circuit that performs automatic power control (hereinafter referred to as APC) such that the semiconductor laser stably emits the laser beam without being affected by the change in temperature when the semiconductor laser forms the latent image on the photosensitive drum.
The latent-image forming unit further includes a pulse width modulation (PWM) circuit that generates a PWM signal to be supplied to the semiconductor laser in order to form a tone image on a transfer sheet of paper.
FIG. 15 is a graph showing the relationship between the pulse width of a PWM signal and the integrated value of optical outputs in PWM drive of the semiconductor laser. A line (1) shows the relationship between the pulse width of the PWM signal and the quantity of light when Ib=0 and bias APC for controlling the emission of the LED light is not performed, a line (2) shows the relationship when 0<Ib<Ith, and a line (3) shows the relationship when Ib=Ith.
In the case of Ib=0 (the line (1)), the semiconductor laser provides no response and the quantity of light is equal to zero when the pulse width of the PWM signal is less than 10%, the semiconductor laser starts to light up when the pulse width of the PWM signal is equal to 10%, the quantity of light linearly increases along with the increase of the pulse width of the PWM signal when the pulse width of the PWM signal is more than 10% and less than 90%, the quantity of light sharply increases when the pulse width of the PWM signal is more than 90%, and the quantity of light is saturated at 100%.
In the case of 0<Ib<Ith (the line (2)), the semiconductor laser provides no response and the quantity of light is equal to zero when the pulse width of the PWM signal is less than 5%, the semiconductor laser starts to light up when the pulse width of the PWM signal is equal to 5%, the quantity of light linearly increases along with the increase of the pulse width of the PWM signal when the pulse width of the PWM signal is more than 5% and less than 90%, the quantity of light sharply increases when the pulse width of the PWM signal is more than 90%, and the quantity of light is saturated at 100%.
In the case of Ib=Ith (the line (3)), the semiconductor laser provides no response and the quantity of light is equal to zero when the pulse width of the PWM signal is less than 5%, the semiconductor laser starts to light up when the pulse width of the PWM signal is equal to 5%, the quantity of light sharply increases along with the increase of the pulse width of the PWM signal, the quantity of light linearly increases along with the increase of the pulse width of the PWM signal when the pulse width of the PWM signal is more than 10% and less than 95%, the quantity of light sharply increases when the pulse width of the PWM signal is more than 95%, and the quantity of light is saturated at 100%.
As disclosed in Japanese Patent Laid-Open No. 7-294837, performing the bias APC for setting the bias current Ib increases the proportion of the linear area of the PWM characteristic to improve the quality of an output image.
However, it is not possible to establish a linear relationship between the pulse width of the PWM signal and the quantity of light in the pulse width from 0% to 100% even in such a PWM circuit in the related art, and dead-band zones appear in areas near a pulse with of 0% and near a pulse width of 100%. Increasing the resolution of the pulse width of the PWM signal and finely controlling an area near 10% where the quantity of light sharply increase and an area near 90% where the quantity of light is toward the saturation in order to avoid the dead-band zone increases the number of input bits in the PWM circuit and the PWM circuit becomes complicated. For example, since it is said that the number of levels of the density gradation which a human being can perceive is about ten in an image having a resolution of 600 dpi, it is sufficient to provide 4-bit density data for an image. However, in order to increase the resolution of the pulse width of the PWM signal for fine control of the semiconductor laser, it is necessary to provide 8-bit density data. Accordingly, there is a problem in that the cost is increased because the number of bits in an image controller that processes image data is unnecessarily increased.
The dead-band zone of the semiconductor laser is determined by the characteristics of the semiconductor laser and the characteristic of a driving circuit therein. With the same semiconductor laser and the driving circuit being used, the proportion of the dead-band zone is increased with an increase of the drive frequency and the linear area is reduced, thus degrading the controllability. For example, a semiconductor laser having the dead-band zone below a pulse width of 5% at a drive frequency of 10 MHz has the dead-band zone below a pulse width of 10% at a drive frequency of 20 MHz.
The tone reproducibility in a lower density area near a pulse width of 0% and in a higher density area near a pulse width of 100% is reduced as the drive frequency of the semiconductor laser is increased along with an increasing need for high-speed processing and for higher image quality of output images and, therefore, the improvement of the image quality of the output images is inhibited.